Preparation of three-dimensional LiMn2O4/carbon composite cathodes via sol-gel impregnation and their electrochemical performance in lithium-ion battery application
A LiMn2O4 (LMO) containing three-dimensional composite cathode was prepared by impregnation of carbon fiber paper with the oxide precursor sol. The sol consisted of the corresponding metal acetates, instead of nitrates in order to prevent the evolution of nitrous gases during the calcination. Poly(vinylpyrrolidone) served as chelating agent, which allows for a homogeneous distribution of the metal ions in the sol. Carbon fiber paper was soaked with the precursor sol and dried repeatedly up to the desired filling grade. After gelation of the precursor sol, the gel was converted to LMO at 450°C. At this temperature transformation of the amorphous gel into crystalline LMO was completed and in the same time it was possible to retain the carbon backbone without significant material damage. The carbon fiber matrix serves as the three-dimensional support for the LMO particles and current collector in the oxide/carbon composite. Microstructural characterization of the LMO/carbon composites revealed that LMO crystals were deposited directly onto the carbon fibers as well as in the voids between the fibers which results in a homogeneous distribution of active material throughout the cathode's bulk. Electrochemical properties of the composite cathodes were investigated as a function of the number of sol impregnation steps. It was noted that the specific capacity of the cathode increases with the number of fillings. Moreover, the irreversible capacity regarding the first charge/discharge cycles was greatly reduced. As proof-of-concept a lithium-ion battery cell containing dry solid polymer electrolyte was assembled and its electrochemical performance was evaluated for future solid-state battery application.